1. Field of the Invention
The present invention relates to a circuit structure and a manufacturing process for a circuit board. More particularly, the present invention relates to a circuit structure and manufacturing process for a circuit board having higher wiring density.
2. Description of Related Art
A conventional circuit board is mainly formed by alternately stacking a plurality of patterned conductive layers and a plurality of dielectric layers, wherein the patterned conductive layers are electrically connected through a plurality of conductive vias. Discriminated based on fabrication processes of the circuit boards, the fabrication processes mainly include a laminating process and a build-up process. Generally, the circuit boards with lower wiring density are mainly fabricated based on the laminating process, and the circuit boards with higher wiring density are mainly fabricated based on the build-up process.
FIGS. 1A-1G are profile flowcharts illustrating a conventional fabrication process for a circuit board. Referring to FIG. 1A, conductive layers 110a and 110b are respectively disposed on two opposite surfaces of a dielectric layer 100, wherein material of the dielectric layer 100 can be epoxy resin or epoxy resin containing glass fiber, and material of the conductive layers 110a and 110b is copper.
Referring to FIG. 1B, a plurality of through holes 112 (only one is illustrated) is formed in the dielectric layer 100 and the conductive layers 110a and 110b, and the way of forming the through holes 112 can be mechanical drilling or laser ablating.
Referring to FIG. 1C, a conductive wall is formed on the surface of the through holes 112 by electroplating, so as to form a conductive through via 114, and meanwhile an electroplating layer is respectively formed on the surfaces of the conductive layers 110a and 110b, and the two electroplating layers are respectively belonged to the conductive layers 110a and 110b. 
Referring to FIG. 1D, the conductive layers 110a and 110b are patterned by a photolithography and etching process, so as to form the circuit patterns.
Referring to FIG. 1E, dielectric layers 120a and 120b are respectively formed on the patterned conductive layers 110a and 110b, and openings 116a and 116b are fabricated on the dielectric layers 120a and 120b by mechanical drilling or laser ablating.
Referring to FIG. 1F, conductive material is filled in the openings 116a and 116b by electroplating, so as to form a plurality of conductive micro via 118a and 118b, and meanwhile conductive layers 130a and 130b are respectively formed on the dielectric layers 120a and 120b, wherein the conductive micro via 118a and 118b, and the un-patterned conductive layers 130a and 130b are formed by electroplating.
Referring to FIG. 1G, the un-patterned conductive layers 130a and 130b are patterned by the photolithography and etching process. Next, a patterned solder mask 140a is formed on the patterned conductive layer 130a, and a plurality of joint pads 142 of the patterned conductive layer 130a is exposed. Moreover, a patterned solder mask 140b is formed on the patterned conductive layer 130b, and a plurality of joint pads 143 of the patterned conductive layer 130b is exposed. Finally, fabrication of a circuit board structure 150 is finished.
According to the above conventional fabrication process of the circuit board, the circuit board is formed by alternately stacking a plurality of the patterned conductive layers and a plurality of the dielectric layers. However, due to limitation of the conventional fabrication process for the circuit board, i.e. limitation of line width and spacing of a fine circuit, a wiring density of the circuit board fabricated based on the aforementioned method cannot be increased. Moreover, during fabrication of the circuit board, the wiring density of the circuit board is directly related to a possible arranging density of the joint pads provided by the circuit board, wherein function of the joint pads is to lap-joint pins of chips, and function as a medium for signal transmission and power supply. Therefore, with gradually increasing trend for number and density of the pins of an integrated circuit (IC) chip, how to provide a higher wiring density on the circuit board has become one of the major R&D directions in fabrication of the circuit boards.